Parasitic capacitances can cause demodulation RFI to differ in inverting and noninverting operational amplifier circuits

The demodulation RFI responses of an inverting operational amplifier (op amp) circuit and a noninverting op amp circuit are compared. The intended voltage gain of the inverting op amp circuit is A₁=-R2/R1=-10. The intended linear voltage gain of the noninverting op amp circuit is A_N1 =(R1+R2)/R1=11. For both circuits, the resistor values are R1=10 kΩ and R2=100 kΩ. Analysis shows that parasitic capacitances C_in (between the inverting and noninverting inputs of the op amp) and CR1 (shunted across R1) cause the inverting op amp circuit to have better RFI immunity than the noninverting op amp circuit. The derivation is based on the hypothesis that the demodulation RFI response is caused by a second-order nonlinearity so that a 3-dB reduction in the linear voltage gain causes the second-order demodulation RFI response characterized by the transfer function H₂ to be reduced by 6 dB. For the assumed values of C_in=8 pF and CR1=0.4 pF. the measured and calculated values of the difference between H₂ values for the inverting and noninverting circuits were in good agreement